1. Field of the Invention
The invention relates in the first place to a throttling device for feeding a liquid under high pressure with a gas dissolved in it into a liquid under lower pressure, which device has a flow orifice between the high-pressure chamber and the lower-pressure chamber, and also downstream of said flow orifice, a body which forms with a wall of the low-pressure chamber a passage slit surrounding the body, through which slit the liquid undergoing pressure release flows, which body is held in an equilibrium state by the interaction of forces across said body as a consequence of the flow through the slit.
2. Description of the Related Bit
Such a device is known from the British Patent No. 1,593,262 and in fact the alternative embodiment, in particular, as described on page 2, left-hand column, lines 2 to 21. This known device consists of a first throttling device which is formed by several orifices in the side wall of a tube with a closed bottom which projects into an orifice of a pipe through which the gas-containing liquid flows. Said tube may have a cylindrical internal wall. In the alternative embodiment mentioned above, said internal wall of the tube becomes wider in a tapered manner in the flow direction and the tube may contain a second throttling device which is formed by a ball or other body which is restrained in a direction opposite to the flow by a spring. The flow through said tube produces a pressure difference across the ball which is under spring pressure, as a result of which interaction of forces a transfer slit is established.
There are very many different processes in which such a feed of liquid under higher pressure into a liquid under lower pressure can occur and in which a very precise control of the flow is necessary. Said liquid under higher pressure may contain particles which may be deposited in the throttling device, as a result of which the throttling action is affected. The optimum action of the throttling device in a particular process is dependent on dimensions, which have to be very precisely adjusted, of the passage through which the pressure release of the liquid under higher pressure to the liquid under lower Pressure has to take place. This is, in articular, the case where the liquid under higher pressure contains a gas which is dissolved in the liquid and which is then released during the pressure release in the throttling device and forms bubbles in the liquid of lower pressure. At the same time, if it is preferred, in the process concerned to obtain a bubble formation in which the small bubbles are extremely small, then the passage of the throttling device must also be very small and the latter readily becomes contaminated. In the above-mentioned known throttling device the first throttling device consists of small orifices in the side walls of the tube, which orifices readily become contaminated. If one of the orifices becomes blocked, then a heavier load is placed on the remaining orifices and this is disadvantageous for the process in which the throttling device is used.
It is known that increase in scale cannot be used with impunity if larger amounts of a liquid have to be fed into the other liquid via a throttling device which employs small throttling orifices. After all, the throttling action changes for a given pressure difference between the two liquids if the throttling orifice alone is enlarged. Increasing the number of small passages to obtain the required flow capacity has, however, the previously mentioned drawback that if a passage becomes blocked, the conditions change in the case of the other passage orifices.
In the cleaning of waste water it is known that water under higher pressure in which a gas, such as air is dissolved is fed into said waste water. During the pressure release of the water under higher pressure via a throttling device into the water under lower pressure, the air may be released from the dissolved state and this takes place with the formation of bubbles, as also described in the previously mentioned British Patent No. 1,593,262. Under these circumstances a cloud of microscopically minute small bubbles should be formed. These rise upwards in the water to be purified and may attach themselves to particles to be separated, which are transported as a result to the surface, where they can be removed by means known per se.
During the formation of bubbles, the surface tension of the water is of importance, since there should be equilibrium between the surface energy of the bubble and the pressure in the bubble.
Small bubbles have the tendency to form large bubbles by coalescence. Large bubbles are, however, undesirable, since the latter rise too rapidly to the surface and do not entrain the particles because they do not attach themselves to them. The problems of the formation of small bubbles in water by causing pressure release of water under higher pressure with air dissolved in it are known from the article entitled "The mechanism of dissolved air flotation for potable water: Basic Analysis and a Proposal" by J. K. Kitchener and R. J. Gochin published in "Water Research", Vol. 15, pages 585 to 590 incl., 1981. It is also known therefrom, however, that there has as yet been no success in forming very minute small bubbles if potable water is involved which, because of its purity, has a high surface tension.
An object of the invention is therefore to provide a throttling device in which the drawbacks of blockage and contamination can no longer occur, which is insensitive to increase in scale and with which it is possible to produce a very fine mist of minute small bubbles in the liquid under lower pressure.
An object of the invention is to provide, in particular, a throttling device which can be expediently used in a liquid purification process, such as a water purification process.
An object of the invention is also to achieve the result that the fine mist of minute small bubbles obtained with the throttling device is maintained.